1. Field of the Invention
The present invention relates to curable compositions for preparing transparent antistatic abrasion-resistant coatings, articles exhibiting antistatic and abrasion-resistance properties coated therewith, in particular optical and ophthalmic lenses for eyeglasses, and a process to prepare such articles.
2. Description of Related Art
It is well known that optical articles, which are essentially composed of insulating materials, have a tendency to get charged with static electricity, especially when they are cleaned in dry conditions by rubbing their surface with a cloth or synthetic piece, for example a polyester piece (triboelectricity). The charges which are present at the surface of said optical articles create an electrostatic field capable of attracting and fixing, as long as the charge remains on optical articles, objects lying in the vicinity thereof (a few centimeters) that have a very little weight, generally small size particles such as dusts.
In order to decrease or suppress attraction of the particles, it is necessary to decrease the intensity of the electrostatic field, i.e. to decrease the number of static charges which are present at the surface of the article. This may be carried out by imparting mobility to the charges, for instance by introducing in the optical article a layer of a material inducing a high mobility of the charges. Materials inducing the highest mobility are conductive materials. Thus, a material having a high conductivity allows dissipating more rapidly charges.
It is known in the art that an optical article acquires antistatic (AS) properties owing to the incorporation at the surface thereof, in the stack of functional coatings, of at least one electrically conductive layer, which is called an antistatic layer. The presence of such a layer in the stack imparts to the article AS properties, even if the AS coating is interleaved between two coatings or two substrates which are not antistatic.
By “antistatic,” it is meant the property of not retaining and/or developing an appreciable electrostatic charge. An article is generally considered to have acceptable antistatic properties when it does not attract or fix dust or small particles after one of its surfaces has been rubbed with an appropriate cloth. It is capable of quickly dissipating accumulated electrostatic charges.
The ability of a glass to evacuate a static charge created by rubbing with a cloth or any other electrostatic charge generation process (charge applied by corona . . . ) can be quantified by measuring the time required for said charge to be dissipated (charge decay time). Thus, antistatic glasses have a discharge time in the order of 100-200 milliseconds, while static glasses have a discharge time in the order of several tens seconds, sometimes even several minutes. A static glass having just been rubbed can thus attract surrounding dusts as long as it requires time to get discharged.
Only a limited number of materials are known in the art for preparing electrically conductive inorganic or organic layers having high optical transparency, i.e. a transmittance in the visible light of at least 90%. Known optically transparent antistatic coatings include vacuum-deposited metal or metal oxide films, for example films based on optionally doped (semi-)conductive metal oxides such as tin oxide doped with indium (ITO), tin oxide doped with antimony (ATO) or vanadium pentoxyde, spin-coated or self-assembled conductive polymer films, spin-coated or extruded carbon nanotube-based composite films.
ITO is the industry standard antistatic agent to provide optically transparent electrically conductive thin coatings, but the performance of ITO suffers when it is applied to plastics. These coatings are fragile and are readily damaged during bending or other stress inducing conditions. Conductive polymers represent the most investigated alternative to ITO coatings, but they still cannot match the optical and electrical performances of ITO and sometimes suffer from thermal and environmental stability problems in specific applications.
Currently, nanocomposites obtained by dispersing carbon nanotubes (CNT) into polymer matrices have brought many promising electrical and mechanical characters in various applications. However, they are still in their infancy and raise a lot of challenges, such as low loading percentage in polymer systems.
Many antistatic polymeric carbon nanotube-based composites have been explored, comprising polymeric resin and electrically conductive carbon fiber/nanotube, or a combination of carbon fiber/nanotube and non-conductive filler. The amount of the electrically conductive filler system utilized is dependent upon the desired electrical conductivity (surface and volume conductivity or resistivity) while preferably preserving intrinsic properties of the polymeric resin such as impact and flex modulus. The polymeric CNT-based composites can be applied in electromagnetic shielding, electrostatic dissipation or antistatic purposes in packaging, electronic components, housings for electronic components and automotive housings.
U.S. Pat. No. 5,908,585 discloses a glass substrate coated with a transparent electrically conductive film obtained from a coating composition containing, based on the total solid content, 0.1 wt % of CNT, 19.9 wt % of conductive nanoparticles of antimony-doped tin oxide and 80 wt % (as SiO2) of hydrolyzed tetraethoxysilane. After high temperature baking at 350° C., the resulting coating is 200 nm-thick and has a surface resistivity of 3·109Ω/with an overall light transmittance of 92% and a haze value of 1.9%. The rest of coatings show even higher haze value than 2%. The abrasion-resistance properties were not investigated.
U.S. patent application No. 2003/158323 discloses an effective dispersion process of CNT into organic polymer matrices such as polyimide or poly(methyl methacrylate) to achieve high retention of optical transparency in the visible range. The final transmittance and the relative optical transparency are still lower than 90%.
U.S. patent application No. 2004/197546 discloses a process to achieve an optically transparent and electrically conductive CNT-based film disposed on a porous membrane through the filtration on said membrane of a dispersion comprising single walled carbon nanotubes and a surfactant or surface stabilizing polymer. However, it is difficult to make such CNT-based film with good quality on curved surfaces, which limits its application in ophthalmic lens industry.
U.S. patent application No. 2005/209392 describes flexible transparent carbon nanotube-based composites films obtained either by first applying a polymer binder onto a transparent substrate, following by a layer of CNT which penetrates into the binder, or by first coating a CNT layer onto the substrate and then applying the polymer binder which diffuses into the CNT network, or a combination of both to form a sandwich structure. The polymer binders can be thermoplastics or thermosets, including silicones, organosilicon polymers, fluorosilicones and inorganic-organic hybrid compounds such as heat-curable silanes, fluorosilanes and metal alkoxides. Although the films having a layer of CNT exhibit light transmittance of about 90-92% at the wavelength of 550 nm and small changes in sheet resistance after having been subjected to an abrasive treatment, the CNT layer show potential high haze after a spray process, due to the absence of binders or surfactants in the CNT dispersion, which is not investigated.
JP2007155802 describes a vacuum deposition process for depositing a thin film using a water repellent composition comprising a conductive material including CNT. The solution to be evaporated comprises large amounts of CNT, typically around 8% by weight. The purpose of this patent application is to apply a water repellent antistatic film. The abrasion resistance is not a specific purpose of the described technique.
The above-mentioned electrically conductive or antistatic coatings have shown very promising performances, but still have limitations with the process, their transparency, or haze values, which prevent them from some specific applications, especially in ophthalmic lens application. In addition, no antistatic coating has been reported to increase abrasion resistance.